1. Technial Field
The present invention relates to platinum alloy electrocatalysts and to acid-electrolyte fuel cell electrodes using the same.
2. Description of the Prior Art
A fuel cell is an electrochemical device for direct conversion of a fuel, such as hydrogen gas and hydrocarbons, and an oxidizing agent, such as oxygen gas, to a low-voltage direct current. It generally comprises a fuel electrode (anode), an oxidizer electrode (cathode), an electrolyte placed between the two electrodes, and means to separately introduce fuel and oxidizer streams to the anode and the cathode, respectively.
In operation, the fuel supplied to the anode is brought into contact with the electrocatalyst and oxidized in the presence of the electrolyte, liberating electrons. The oxidizing agent, on the other hand, is fed to the cathode, where it is reduced on the surface of electrocatalyst in the presence of the electrolyte, consuming the electrons transferred from the anode via an external circuit and generating the electric power.
As is apparent from the above, a fuel cell requires electrocatalysts for both the anode and cathode. It is known that, of the Group-8 metals of the Periodic Table (Fe, Ru, Os), Group-9 metals (Co, Rh, Ir) and Group-10 metals (Ni, Pd, Pt), the "platinum group metals" (Pt, Pd, Rh, Ru, Ir and Os) can be advantageously used, either alone or in combination, as the electrocatalyst. It is common practice that such a platinum group metal, or a combination thereof, is supported on a conductive carrier material, such as conductive carbon black, in a well dispersed form and the catalyst thus obtained is fixed to a support member, such as wetproof graphite paper, thus making up an electrode.
The output efficiency of a fuel cell is dictated by a number of factors, but its dependency upon the activity and service life of the cathode catalyst is by far the most outstanding. It is well known that in oxygen-hydrogen feed phosphoric acid fuel cells, for example, the activation polarization of oxygen reduction at the cathode is far larger than that of hydrogen oxidation at the anode. When an electrocatalyst supporting a platinum group metal (for example platinum) is used as cathode, sintering or growth of platinum crystallites tend to progress during cell operation, significantly decreasing the active surface area of the metal catalyst, which can lead to reduction in the cell output and in overall operation efficiency.
To eliminate such difficulties, a wide variety of studies have been made on supported metal catalysts. These include alloys of a platinum group metal with various other metals, primarily Group 2 to 6 base metals such as vanadium, tungsten, aluminum, titanium, silicon, cerium, strontium and chromium (U.S. Pat. Nos. 4,186,110, 4,202,934 and 4,316,944 ); ternary alloys prepared by adding cobalt to platinum-vanadium or platinum-chromium alloys (U.S. Pat. No. 4,447,506 ); and alloys of a platinum group metal with gallium, or superlattice alloys between a platinum group metal and iron [Japanese Patent Application Laid-open Nos. 7941 and 156551(1985 )].
None of these catalysts, however, is completely satsifactory, leaving much room for further study; some of these have a sufficiently high initial activity, but tend to lose their activity in a relatively short time, and others retain their activity for long periods, but the level of activity is not sufficiently high.